Field of the Invention
The present invention relates to curing systems for compositions containing halogenated isomonoolefin/para-alkylstyrene copolymer elastomers and blends thereof with other polymers.
Description of Related Art
It is known in the prior art that a combination of a zinc salt of acrylic or methacrylic acid and an organic peroxide may be used to cure rubber compositions containing saturated or unsaturated elastomer compositions and blends thereof.
For example, U.S. Pat. No. 4,713,409 discloses vulcanizable polymer compositions comprising a rubbery polymer such as natural rubber, ethylene/propylene rubber, ethylene/propylene/diene rubber, GR-S, nitrile rubber, neopyryene rubber and blends thereof cured using a combination of at least 25 parts by weight per hundred parts rubber (phr) of a zinc dimethacrylate and a cure effective amount of a peroxide curing agent.
Similarly, U.S. Pat. No. 4,857,571 discloses a free radical curing system for saturated and unsaturated elastomer compositions which is based on a combination of a free radical generating agent such as an organic peroxide and a polyfunctional monomer co-agent which is capable of reacting with the free radical generator to crosslink the elastomer composition. Similar compositions are also disclosed in U.S. Pat. No. 5,288,446.
U.S. Pat. No. 4,987,192 discloses colored tire stocks which are preferably based on a blend of chlorobutyl rubber, natural rubber and EPDM rubbers which are both covalently and ionically crosslinked using a curing system containing conventional sulfur-containing curatives in combination with a zinc dimethacrylate and an organic peroxide.
Crosslinking in these and similar systems generally relies on the creation of free radical sites along the polymer chain and the addition of the difunctional acrylate or methacrylate monomer at these sites to effect crosslinking.
U.S. Pat. No. 5,162,445 discloses halogenated elastomeric random copolymers of an isomonoolefin such as polyisobutylene and a para-alkylstyrene such as para-methylstyrene, referred to herein as BIMS rubber. These copolymers are devoid of ethylenic unsaturation thereby enhancing their resistance to heat, solvents and ozone and may be readily crosslinked by nucleophilic or ionic reactions involving the benzylic halide group present along the polymer chain. Suitable crosslinking agents include a combination of zinc oxide and stearic acid, or zinc stearate alone used at level above about 3.0 parts by weight per hundred parts rubber (phr). Zinc diethyldithiocarbamate is also an effective curative for BIMS rubber. Zinc stearate is also known as an effective curative for elastomer compositions based on a blend of the BIMS rubber and unsaturated diolefin rubbers such as polyisoprene, natural rubber or polybutadiene, as disclosed in copending U.S. application Ser. No. 08/433,773, filed May 3, 1995. The cure level of zinc stearate used in such curable blends ranges from about 0.05 to less than 3 parts by weight phr.
One of the major goals in exploring curing systems for BIMS elastomers is to uncover curatives which will not only effectively cure the BIMS elastomer by itself, but will also cure blends of BIMS elastomers with other chemically diverse saturated polymers such that a relatively balanced cure for each individual elastomeric component of such blends is achieved.